At the end of the day, many people won't have the objectivity required to make hard decisions when it comes to a family member: we see it often enough in the news that someone decides to prolong the life of a child or significant other despite the latter being in pain and the doctor advising against it.
They've trained for years, they understand the ramifications and possibilities for successful life etc. much more than your average Joe does, so let them do their job.
However, in cases where the doctor is also unsure and cannot give clear advice (comas or potential loss of limb for example), the family should make the decision.
Relatives, the doctor should not decide himself. In this situation, the relatives should decide because the relatives are the ones closest to the one in question. The doctor should only decide himself if there are no relatives present or if the relatives let the doctor decide. These words are filler.